Key points

MI6, headed by Sir John Sawers, is responsible for Britain's overseas intelligence.

The Government Communications Headquarters, headed by Sir Iain Lobban, is responsible for Britain's signals intelligence.

The spy chiefs took questions for 90 minutes from a parliamentary committee, and did not hold back when asked about the US whistleblower.

Sir Iain Lobban, the head of GCHQ, says terrorist groups in the Middle East are directly discussing certain elements of the leaks.

"We've seen terrorist groups in the Middle East, in Afghanistan and elsewhere in south Asia discussing the revelations in specific terms, in terms of the communications packages that they use, the communication packages that they wish to move to," he said.

Barbara Miller analysis

At the admission of one of the members of the Intelligence and Security Committee, the first 20 minutes of this unprecedented hearing was a little stilted.

That was classic British understatement.

If any member of the general public had inadvertently stumbled upon this strange daytime TV offering they would have reached for the remote long before it got halfway interesting.

It took the best part of an hour to really get into the issue on everyone's mind: Edward Snowden's security leaks.

It was only when talking of the damage they feel the former intelligence contractor has done that some kind of feeling broke into the spooks' calm, polished, and, at times, clearly rehearsed performance.

Terrorists rubbing their hands with glee, Al-Qaeda lapping it up, evidence of chatter from Afghanistan to the Middle East that suggests terrorists are changing their modus operandi in light of the secrets leaked by the Edward Snowden - it was colourful stuff.

Earlier, the journalist who facilitated some of that leaking, Glenn Greenwald, had berated a BBC journalist for, he said, repeating things government officials whisper into the media's ears but provide no proof for about the negative consequences of leaking information.

The head of GCHQ, Sir Iain Lobban, said he wouldn't "compound the damage" by being more specific about the intelligence, suggesting the whistleblower had made the agency's job "far, far harder for years to come".

Sir Iain said he could do so in a private hearing.

That won't be enough to satisfy the sceptics, and therein lies the catch-22 of this whole exercise in apparent transparency.

"I can say that explicitly and so the cumulative effect of the media coverage, global media coverage, will make the job that we have far, far harder for years to come."

Sir John Sawers, the head of MI6, says the Snowden leaks have put operations at risk.

"The leaks from Snowden have been very damaging. They've put our operations at risk," he said.

"It is clear that our adversaries are rubbing their hands with glee - Al Qaeda is lapping it up."

The session was more of a wide-ranging discussion than a gilling, and many issues were not dealt with in great detail.

It was broadcast with a two-minute delay for fear that the men may let something important slip during.

However, there was a sense that the men relished the chance to speak about what they think of the Snowden revelations.

They rejected suggestions that millions of ordinary citizens were being actively spied upon.

"We do not spend our time listening to the telephone calls or reading the emails of the majority, of the vast majority. That would not be, proportionately - it would not be legal. We do not do it," Sir Iain said.

Sir Iain also emphasised that just because the spy services remain secretive, it does not mean they are beyond accountability.

"I don't think secret means unaccountable in any sense. I don't think secret means sinister, I'd like to hammer that home," he said.